The variables are rain and grazing through the seasons plus a few physical barriers.
Refer to the maps. Outlines of Kenya and Tanzania. Then the larger Serengeti Ecosystem map. Note the Serengeti Plains in the south. The western corridor in the west. The Lobo Hills in the north. Seronera in the center. The Masai Mara is in the far north of the Serengeti-Mara ecosystem.
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Kenya Great Migration Map
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Tanzania Wildebeest Great Migration Map
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Let’s start the annual cycle with the “short” and light rains in November and December (sometimes as early as October).
• The rains fall on fertile volcanic soils which are remnants of the volcanoes in the southern Ngorongoro Conservation Area.
• The result is short sweet grass which draws the migration rapidly south from Kenya’s Masai Mara. The migration moves down the eastern side of Tanzania’s Serengeti into these sweet short-grass plains.
• The wildebeest settle in the southern plains between January and April as there’s lots of food.
• In late March or April and May the “long” or heavy rains set in. The depleted southern plains are less attractive than the long grass plains up in the western corridor. As a result, the wildebeest migration moves north westerly.
• Large river crossings on the Grumeti and Mara Rivers occur as the migration heads back north towards the Mara. The season dries out and fresh grazing and water can be found in the far north. The Mara is usually at its best in August, September, and October especially when it’s very dry.
• Fresh rains start building around October into November. The migration gets restless as it anticipates the change in season. It moves north and south and back again. This is when we usually get the best river-crossing action.
• The cycle starts again when the short rains break and result in fresh sweet grass in the southern Serengeti plains. The wildebeest migration moves rapidly south.
How the migration really works!
The theory is simple. Seasonal rains and the availability of grazing determines the “clockwise” movement of the migration. The larger ecosystem includes Tanzania’s Serengeti and Kenya’s Masai Mara. A few physical barriers like the Simiti and Lobo hills and the Grumeti and Mara rivers hinder and alter this “circular” path. Well in reality it’s not quite that simple!
Here’re some notes gleaned from Richard Knocker. One of the original Nomad Guides in East Africa.
Between the short and long rains – November to April
The wildebeest want to be in the short grass plains of the southern Serengeti. That’s near Ndutu, Gol, and Southern Loliondo, but the water and grazing cannot support them all year round. This is where they choose to give birth to their young with rich grass to support them. That’s usually February and March. Within a short space of time, perhaps 4 to 6 weeks, several hundred thousand calves are born. This is where and when we see much of the dramatic predator action.
The wildebeest migration moves off in search of sustenance in response to periods of dry weather. They leave this sweet area as late as possible and come back as soon as they can. But the rains are unpredictable. So every year is different and, in fact, every week can be different.
The migration is also not a continually forward motion. They go forward, back and to the sides, they mill around, they split up, they join forces, they walk in a line, they spread out, and they hang around. You can never predict with certainty where they will be.
So, soon after the short rains start we expect the migration to be in the sweet grass plains area around Naabi, Ndutu and Gol. That’s from December through to April. Depending on local rainfall, they might be anywhere from Moru Kopjes through to the slopes of Ngorongoro.
After the long rains into the dry season – May to October
From May, the rains stop and the herds gradually start moving. Usually, as the plains of the south and east dry out, there is a movement to the north and west. That’s because there’s more grass and more dependable water.
Not all the wildebeest and zebra will follow the same route though. This means that while part of the migration will head to the western corridor and the Grumeti River before heading north, significant numbers may also go up through Loliondo, or via Seronera and Lobo.
In a dry year, the first wildebeest could be near the Mara River in early July as this is the only decent permanent water in the ecosystem. In a wet year, as late as mid-August. If conditions are very good, with plenty of grass and water the herds will be spread out all the way from Seronera to the Mara River.
The wildebeest migration as a whole need not all pass into Kenya. Many stay behind in Tanzania and then cross and re-cross the border areas. This carries on until October into November when they start thinking of heading back. Again this will be dependent on the rain.
Some notes on the Mara “river crossings”
• The Mara river crossings happen at any time during the dry season. But they’re elusive, they happen quickly. They’re unforgettable experiences.
• The areas that the migration wildebeest covers are vast, even when crossed in a 4WD car. That’s in both the Serengeti and the Mara.
• Note that there’s more Mara River frontage in Tanzania than there is in Kenya. The migration can criss-cross over the border. People can’t!
• The migrating groups may be split over a wide area. Finding one on the brink of crossing is not a given.
• The wildebeest are easily spooked by real or imagined threats. They fear crossing the river, as they have an inkling that something lurks there.
• Patients waiting near a herd by the river may only produce a puff of dust as they turn on their heels and run away. Or maybe the herd is just not ready to cross the river and they are milling around contentedly.
• However, if everything is right. Then there is utter and extraordinary chaos as the herds struggle to get to the other side of a major river filled with crocodiles.
The migration starts moving south as the new season’s rains start.